Individuals' mental health critically depends on having positive social relationships. Although an enormous amount of research has focused on the mental-health benefits of positive social relationships, much less is known about the predictors of such beneficial social relationships. Specifically, extant research is agnostic about what motivates individuals to cultivate positive social relationships. In this proposal, we suggest that individuals who experience high levels of vicarious reward (i.e., subjective reward in response to others' positive outcomes) will be most likely to seek out and maintain positive social relationships by acting prosocially toward others. Frequent prosocial behavior, in turn, may buffer these individuals from stress and negative emotions. In fact, previous work suggests that engaging more frequently in prosocial behavior reduces mental health symptoms. In other words, individuals who are sensitive to vicarious reward may behave in ways that foster positive relationships and the beneficial, downstream effects those relationships provide. In order to test our hypotheses, we will employ a joint neuroimaging-daily experience sampling study (Study 1). In the neuroimaging portion of Study 1, we will measure neural activity as participants win money for themselves (personal reward) and when they observe another person win money (vicarious reward). The same participants will then complete a 21-day diary study, indicating how often they engaged in prosocial behaviors (e.g. providing support to others) and experienced stress and negative emotions. Next, we will characterize the neural networks underlying personal and vicarious reward and assess how these two types of reward track together within individuals (Aim 1a). We will then test the hypotheses that individual differences in neural reward sensitivity to vicarious reward will predict frequency of daily prosocial behavior (Aim 1b), and that frequently engaging in prosocial behavior will reduce daily stress and negative emotions (Aim 1c). This multi-method approach will assess if vicarious reward serves as a neural marker of positive social relationships. To complement this individual differences approach, a follow-up neuroimaging study (Study 2) will examine whether sensitivity to vicarious reward can be manipulated within individuals (Aim 2). New participants will meet two individuals before entering the scanner and be asked to write a passage about one individual in a first-person perspective and the other individual in a third-person perspective. After entering the scanner, participants will win money and observe these two individuals win money. We predict that adopting a first- person perspective should heighten neural sensitivity to vicarious reward (but not personal reward) for that specific other person. If this is the case, Study 2 will provide early suggestions on how to shift a potential neural marker of social well being (i.e., vicarious reward) within individuals. By testing this model of social affect and its relationship to mental health, we hope to suggest novel ways to improve mental health outcomes by internally motivating individuals to engage in beneficial social behaviors.